1. Field of the Invention
The present invention relates to an optical encoder for converting mechanical displacement into an electrical signal.
2. Description of the Related Art
With reference to FIG. 13, a conventional exemplary optical encoder will be described.
FIG. 13 is a schematic diagram showing a conventional optical encoder. A conventional encoder 100 includes: a light source 101; a collimator lens 102; a fixed slit plate 103; a movable slit plate 104; and a light-receiving portion 105. The collimator lens 102 is used for collimating a light beam emitted from the light source 101. After traveling through the collimator lens 102, the light beam enters the slit plates 103 and 104. The fixed slit plate 103 and the movable slit plate 104 respectively have a plurality of slits which are equal in pitch and parallel to each other. The light-receiving portion 105 receives a flux of light passing through the fixed slit plate 103 and the movable slit plate 104, and converts the flux of light into an electric signal.
The light source 101, the collimator lens 102, the movable slit plate 104 and the light-receiving portion 105 are placed within a movable section 106. The movable section 106 moves in a direction parallel to a face of the fixed slit plate 103 on which the slits are formed and vertical to the direction of slits.
Hereinafter, the operation of the conventional optical encoder 100 having the above-mentioned configuration will be described.
After being collimated by the collimator lens 102, the fixed slit plate 103 is irradiated with a light beam emitted from the light source 101. Since the slits on the fixed slit plate 103 and the movable slit plate 104 are equal in pitch and parallel to each other, the light beam entering the movable slit plate 104 is transmitted or shielded depending on the relative position of the slits. Since the movable slit plate 104 is placed on the movable section 106 which moves in a direction parallel to the slit of the fixed slit plate 103, the amount of light going out from the movable slit plate 104 changes depending on the displacement of the movable section 106. The light-receiving portion 105, which is placed so as to face the side of the movable slit plate 104 from which the light beam goes out, converts the change in the amount of light into an electric signal, thereby detecting the amount of displacement of the movable section 106.
In the conventional optical encoder 100 thus configured, the slit pitch should be reduced in order to detect the position with higher precision. If the slit pitch is reduced, however, the effect of diffraction is increased, resulting in the reduced change in the amount of transmitted light which occurs due to the displacement of the relative position between the slit plates 103 and 104. As a result, difficulty arises in detecting the signal. The effect of diffraction of light can be reduced by shortening the distance between the slit plates 103 and 104. In this case, however, there arises a problem that the slit plates 103 and 104 are likely to be broken with ease when they are brought into contact with each other due to impact or vibration. For example, assuming that resolution of the positional detection is 5 .mu.m per pulse of the signal, the slit pitch of the slit plates 103 and 104 is 5 .mu.m. In this case, it is necessary to set the distance between the slit plates 103 and 104 to several .mu.m or less in order to obtain sufficient change in the light intensity at the light-receiving portion 105.
It is known that when the slits having a pitch p are irradiated with light using the light source having a small wavelength width and height coherence (having a wavelength of .lambda.), the dark and bright patterns having the same pitch as the slit pitch which are called Fourier images, are generated at the positions expressed by m.times.(p.times.p/.lambda.) (m is an integer) in the rear of the slit. The use of these Fourier images makes it possible to increase the distance between the slits without decreasing the amount of change in the light intensity at the light-receiving portion 105. In the case where the slit pitch is reduced to 5 .mu.m, however, the distance between the Fourier images is also reduced. For example, assuming that the wavelength of the light source is 780 nm, the Fourier images are generated at intervals of 32 .mu.m from the slit plate 103. If the position of the slit plate 104 deviates from the positions of the Fourier images due to the change in the distance between the slit plates 103 and 104, the amount of the light intensity at the light-receiving portion 105 is reduced. Therefore, the variation in the distance between the slit plates 103 and 104 should be within about 8 .mu.m. Moreover, since the odd-numbered Fourier images and the even-numbered Fourier images have reversed patterns of dark and bright, it is necessary to precisely place the two slit plates 103 and 104 so as to be parallel to each other. In this way, in the conventional optical encoder 100, it is necessary to precisely adjust the arrangement of the two slits 103 and 104.